APOGEE Overview

Apache Point Observatory Galactic Evolution Experiment (APOGEE)
APOGEE Targets

DR13 includes data for ~163,000 APOGEE targets. This includes 146,000 science targets, located in distinct types of survey fields:

~15,000 stars in Bulge fields
~28,000 stars in Halo fields
~55,000 stars in Disk fields
~14,000 stars in Kepler/CoRoT fields
~8,000 objects in Ancillary Science fields
~1,800 stars in Halo Stream fields
~1,200 stars in Sagittarius dSph fields
~8,000 stars in Star Cluster fields
~900 bright stars observed with the NMSU 1m telescope + APOGEE, including bright standards

The APOGEE main survey sample also includes ~17,000 hot stars used for telluric correction, across all field types.

Data Release 13 includes spectra, derived stellar parameters, and elemental abundances for more than 100,000 stars, sampling all major components of the Milky Way.

Data Release 13 (DR13) is the third spectroscopic release from the Apache Point Observatory Galactic Evolution Experiment (APOGEE/APOGEE2). DR13 releases the same data as were previously released in DR12, but with modifications to the processing, analysis, and calibration.

By operating in the infrared portion (H-band, details of wavelength coverage) of the electromagnetic spectrum, APOGEE is better able to detect light from stars lying in dusty regions of the Milky Way than surveys conducted in the optical, which makes this survey particularly well-suited for exploring the Galactic disk and bulge. APOGEE’s high resolution spectra (R~22500) provide detailed information about the properties of individual stars. DR13 provides APOGEE’s processed infrared spectra, as well as catalogs of radial velocities, stellar parameters, and abundances derived from these spectra.

DR13 provides APOGEE calibrated abundance measurements of 23 individual species: C, CI, N, O, Na, Mg, Al, Si, P, S, K, Ca, Ti, TiII, V, Cr, Mn, Fe, Co, Ni, Cu, Ge, and Rb.

DR13 Scope and Status

The APOGEE survey started in September 2011, and DR13 includes all APOGEE observations through July 2014 (encompassing all of SDSS-III). By design, spectra for most of the survey stars are integrated over multiple visits, with at least one visit separated by at least one month, to enable the identification of binary stars through detection of radial velocity variations.

In DR13, data for all stars observed during SDSS-III/APOGEE-1 are being re-released using improved data processing and abundance pipelines. Included are 12,000 targets observed while commissioning the APOGEE-North spectrograph (most of these objects were re-observed with the spectrograph in its final, SDSS-III survey configuration) and ~900 stars that were obtained using the NMSU 1-m telescope at Apache Point Observatory, using a fiber link to the APOGEE instrument. Spectra of brighter stars can be taken with the 1-m telescope allowing observations of calibration objects, and various ancillary program objects.

The data released in DR13 are the same data as were previously released in DR12. However, the processing and analysis have been modified in several ways:

  • the processing of the spectra improves the correction for telluric absorption
  • the linelist used for determining stellar parameters and abundances has been revised
  • abundances are derived for several more elements than in DR12 (CI, P, TiII, Co, Cu, Ge, Rb), although uncertainties may be significant for some of these, and not all have been calibrated
  • results are available for cooler stars using a newly-employed MARCS model atmosphere grid (note that cool stars still present significant challenges; only raw [uncalibrated] results are available for the coolest giants)
  • separate synthetic spectral grids are used for dwarfs and giants; different isotope ratios are used for dwarfs and giants. The grids for dwarfs include rotation.
  • some attempt to account for variable line spread functions (LSFs) for different objects has been made
  • the calibration of elemental abundances has been modified, and the zeropoints of elemental abundances have been adjusted in a final calibration step
  • some modifications have been made in how the parameters and abundances are presented in the data files and tables

Since 2014, APOGEE data collection has continued as part of the SDSS-IV/APOGEE-2 project. APOGEE-2 will provide additional data collected from the SDSS telescope at Apache Point Observatory; but starting in late 2016, it will also include data collected from the duPont telescope at the Las Campanas Observatory. APOGEE-2 data will be distributed in subsequent data releases.

The coverage diagrams below show the areas of sky with APOGEE data in DR13. The figures show the Milky Way, centered at the center of the Galaxy. APOGEE pointings (areas of observations) are shown a small circles.

Distribution of observed APOGEE fields, color-coded by the number of approximately 1-hr visits.
Distribution of observed APOGEE fields, color-coded by the number of approximately 1-hr visits.
Distribution of APOGEE survey and commissioning fields, and, for the former, whether the survey observations were completed. Most commissioning observations were repeated during the main survey with the spectrograph in its survey configuration.
Distribution of APOGEE survey and commissioning fields, and, for the former, whether the survey observations were completed. Most commissioning observations were repeated during the main survey with the spectrograph in its survey configuration.

Data Access

Several interfaces are available to access the data (see APOGEE Data Access for more details):

Catalog Archive Server (CAS)

APOGEE target information and derived radial velocities, stellar parameters, and abundances are loaded into the Catalog Archive Server (CAS) database.  This contains a number of ways of interfacing with the database, including low-level SQL access through the CasJobs interface that allows you to save and analyze all your search results.

Science Archive Server (SAS)

Summary FITS files containing the derived radial velocities, stellar parameters, and abundances for all APOGEE stars, as well as all raw and processed APOGEE data are available from the Science Archive Server (SAS). A web interface allows users to retrieve spectra for either individual objects or in bulk.

SkyServer

The SkyServer Explore tool provides a quick way to find APOGEE spectra for a desired object. Quick Look shows an image and spectrum for all SDSS objects. For objects with APOGEE spectra, the tool includes an APOGEE section. The APOGEE section of the Explore tool shows a composite spectrum, radial velocities, and derived stellar atmospheric parameters, and gives links to further data, including the spectrum as a FITS file.

All APOGEE catalog data are available through the search tools of SkyServer. The Infrared Spectroscopy Query Form lets you search for APOGEE catalog objects by position, spectral classification, redshift, and other constraints in spectroscopy and/or imaging. SQL Search lets you create your own search using the SQL database programming language; see the SkyServer SQL Tutorial to learn how to write SQL queries.

Additional Information

Broadly speaking, the main APOGEE data products released in DR13 are the outputs of two software pipelines. The data reduction pipeline delivers extracted, one-dimensional, calibrated spectra as well as derived radial velocity information. Those interested in working with APOGEE spectra should be aware of instrumental and reduction-specific features visible in the data. Users of the released spectra are strongly advised to visit the Using APOGEE Spectra page.

The APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP) determines stellar parameters and abundances through the automatic analysis of APOGEE’s high-resolution H-band spectra. ASPCAP delivers derived effective temperatures (Teff), surface gravities (log g), and 23 chemical elements. Users interested in working with ASPCAP data are strongly advised to read the documentation in the Using APOGEE Stellar Parameters page and the Using APOGEE Chemical Abundances page, where uncertainties, potential systematic effects, and other important issues are discussed.

Detailed APOGEE Information

The following pages give detailed information about various aspects of APOGEE.

The APOGEE instrument and survey
gives an overview of how APOGEE spectroscopic data are taken and organized.
Catalogs
describes the APOGEE information that is stored within summary data files and the CAS database and how it can be used. This includes some examples of how to query the database or summary data files to select out different pieces of information for different types of targets.
APOGEE Data Access
describes all of the APOGEE data products that are available through the Science Archive Server.
Using APOGEE Stellar Parameters
describes important things you need to know if you plan to use the derived stellar atmospheric parameters.
Using APOGEE Chemical Abundances
describes important things you need to know if you plan to use the derived elemental abundances.
Using APOGEE Spectra
describes some important features in APOGEE spectra about which anyone looking at spectra should be aware.

Detailed APOGEE Software Pipeline Steps

The following pages describe the steps in the APOGEE software pipelines, and the files created at each step.

APOGEE Target Information
describes the way in which APOGEE targets are chosen (targeted), and how this is documented in the target flags.
APOGEE Visit Spectra Reduction
gives information about how individual visit spectra are observed, processed and stored.
APOGEE Visit Spectra Combination
gives information about how the combined spectra for each star are created and stored, including information about the derivation of radial velocities.
APOGEE Stellar Parameter and Abundance Determination
describes how the stellar parameters and abundances available in APOGEE spectroscopic catalogs are derived.
APOGEE Caveats
is a running list of known issues with the DR13 release.

Technical papers

These web pages attempt to describe the key features in APOGEE data, but they must necessarily be somewhat concise. For the most in-depth discussion of the APOGEE survey and data analysis, users should consult the APOGEE technical papers, which include:

The APOGEE overview paper
Majewski et al. 2015 discusses the scientific motivation for the survey, as well as the survey requirements and the choice of survey field placements that are implied. It describes survey operations, summarizes the level to which requirements are met, and references the data releases. It also introduces the follow-on APOGEE-2 survey.
The APOGEE DR12 data paper
Holtzman et al. 2015 describes the DR12 release of SDSS-III/APOGEE data. While there are some modifications for DR13, it presents a general discussion of the calibration of the stellar parameters and abundances, and validation of these relative to independent measurements. It also presents the methods by which APOGEE data can be accessed, and discusses some details of the files and databases, including important information to users about what quantities are stored where, uncertainties, and potential issues.
The APOGEE instrument paper
(Wilson et al. 2016, in prep) discusses the technical details of the instrument itself, and presents some low level instrument performance data.
The APOGEE target selection paper
Zasowski et al. 2013 discusses the target selection for main survey and APOGEE-1 ancillary science projects. An expanded discussion of the Kepler field targeting can be found in Pinsoneault et al. 2014.
The APOGEE data reduction paper
Nidever et al. 2015 discusses the data reduction pipeline, describing how the raw data are analyzed to produce reduced, calibrated spectra. It also presents additional instrument performance data (flats, darks, LSF, persistence, etc.), It also discusses the measurement of radial velocities and their quality.
The APOGEE/ASPCAP paper
Garcia Perez et al. 2016 presents an outline of how the spectra are processed and analyzed to produce stellar parameters and abundances. It demonstrates validation of the overall method using recovery of parameters/abundances from simulated data, and discusses uncertainties that are introduced by real-world issues: S/N, issues related computational efficiency, variation and uncertainty of the LSF, and issues involving the loss of information under sky lines. It presents some basic tests of the methodology from very-high-resolution observations of some well-studied stars (FTS stars).
The APOGEE linelist paper
Shetrone et al. 2015 presents the details of how the H-band linelists, a critical component for ASPCAP, were developed.

  • The APOGEE linelists are tested using high-resolution IR spectra of several well-studied stars in Smith et al. (2013).
  • Further tests of the individual elements derived from the ASPCAP pipeline are found in Cunha et al. (2015).
The APOGEE model atmospheres paper
Meszaros et al. 2012 discusses details of the model atmospheres that were used for the final APOGEE-1 analysis, which is included in DR13.
The APOGEE spectral grids paper
Zamora et al. 2015 presents how the spectral synthesis was done, documents the libraries that have been used, and investigates the sensitivity of the result to the choice of synthesis code and model atmospheres.
The APOGEE DR10 calibration paper
Meszaros et al. 2013 discusses calibration of stellar parameters that were released in DR10. It is superseded by the APOGEE DR12 data paper ( Holtzman et al. 2015 ).

Additional technical papers on APOGEE may be forthcoming.